In recent years, LCD panels have been widely used for flat panel displays. However, such panels still have several drawbacks such as slow response time and narrow viewing angle. In addition, even in many new systems in which these drawbacks are redressed, there still remain several problems, including unsatisfactory performance and increasing costs in the manufacturing of panels. In these circumstances, thin film EL devices are attracting attention as new light-emitting devices that have excellent visibility because of self-luminosity, high-speed response, and widespread applicability. In particular, organic EL devices, thin film EL devices that use, in all or part of the layers, organic materials, allowing for a simple film-forming step such as vapor deposition or coating at room temperature, have been the focus of much research, as these devices are attractive in terms of manufacturing cost as well as the above-mentioned characteristics.
In thin film EL devices (organic EL devices), the light emission arises from the recombination of electrons and holes injected from electrodes. Research on such devices has long been conducted; however, since the electroluminescent efficiency of these devices was generally low, their practical applications for light emitting devices was still a long way off.
In the meantime, a device was proposed by Tang et al. in 1987 (C. W Tang and S. A. Vanslyke, Appl. Phys. Lett., 51, 1987, pp. 913.) comprising a hole-injecting electrode (anode), a hole-transporting layer, a luminescent layer, and an electron-injecting electrode (cathode) on a transparent substrate wherein ITO (Indium Tin Oxide) was employed as the anode, a 75-nm-thick layer of diamine derivative as the hole-transporting layer, a 60-nm-thick layer of aluminum quinoline complex as the luminescent layer, and an MgAg alloy having electron-injection properties and stability as the cathode. This device not only made improvement in the cathode but also formed a thin film which had satisfactory transparency even with a film thickness of 75 nm and which was uniform and free from pinholes and the like by employing a diamine derivative, having excellent transparency, for the hole-transporting layer. Thus, because reduction in the device's total film thickness became possible, light emission having high luminance with relatively low voltages could be achieved. Specifically, with a low voltage of 10 V or less the device achieved a high luminance of 1000 cd/m2 or more and a high efficiency of 1.5 lm/W or higher. This report led by Tang et al. spurred further investigation into improvements in cathodes, suggestions on device constructions, and so forth, and this active investigation has continued to the present.
Thin film EL devices, generally investigated today, are outlined below.
In addition to a thin film EL device, such as one described in the above-mentioned report, having a laminate structure of an anode, a hole-transport layer, a luminescent layer, and a cathode formed on a transparent substrate, a device may comprise a hole-injecting layer formed between an anode and a hole-transport layer, may comprise an electron-transport layer formed between a luminescent layer and a cathode, or may comprise an electron-injecting layer formed between the electron-transport layer and the cathode. Thus, by assigning functions to each individual layer separately, it becomes possible to select suitable materials for each layer, resulting in improvement in device characteristics.
For the transparent substrate, a glass substrate such as Corning 1737 is widely used. A substrate thickness of about 0.7 mm is convenient for use in terms of its strength and weight.
For the anode, a transparent electrode such as an ITO-sputtered film, an electron-beam evaporated film, or an ion-plated film is used. The film thickness is determined by the sheet resistance and visible light transmittance required; however, since thin film EL devices have relatively high operating current densities, in most cases, the film thicknesses are made to be 100 nm or more so as to reduce the sheet resistances.
For the cathode, an alloy of a low work function metal with a low electron injection barrier and a relatively high work function, stable metal, such as an MgAg alloy proposed by Tang et al. or an AlLi alloy, is used.
For the layers sandwiched between the anode and the cathode, many devices have a laminate structure, for example, of a hole-transport layer formed to a thickness of about 80 nm by vacuum vapor deposition of a diamine derivative (Q1-G-Q2 structure) used by Tang et al. such as N,N′-bis (3-methylphenyl)-N,N′-diphenylbenzidine (TPD) or N,N′-bis (α-naphthyl) -N,N′-diphenylbenzidine (NPD) and a luminescent layer formed to a thickness of about 40 nm by vacuum vapor deposition of an electron-transport luminescent material such as tris(8-quinolinolato) aluminum. In this structure, in order to increase luminance, generally, a luminescent layer is doped with a luminescent dye.
In addition, in view of the general difficulty in obtaining an organic compound having excellent electron-transport properties such as one described above, it has also been suggested that in the luminescent layer/electron-transport layer structure and in the hole-transport layer/luminescent layer/electron-transport layer structure a hole-transport luminescent material be used for the luminescent layer.
For example, Japanese Unexamined Patent Publication No. 2-250292 discloses a device having the hole-transport luminescent layer/electron-transport layer structure that uses, as the hole-transport luminescent material, [4-{2-(naphthalene-1-yl)vinyl}phenyl]bis(4-methoxyphenyl)amine or [4-(2,2-diphenylvinyl)phenyl]bis(4-methoxyphenyl) amine.
International Patent Publication No. WO96/22273 discloses a device having the hole-transport layer/hole-transport luminescent layer/electron-transport layer structure that uses, as the hole-transport luminescent material, 4,4′-bis(2,2-diphenyl-1-vinyl)-1,1′-biphenyl.
At the 1998 MRS Spring Meeting, Symposium G2.1, the hole-injecting layer/hole-transport luminescent layer/hole blocking layer/electron-transport layer structure that uses NPD as the hole-transport luminescent material was disclosed.
Further, Japanese Unexamined Patent Publications No. 10-72580 and No. 11-74079 also disclose various hole-transport luminescent materials.
Thus, using a hole-transport luminescent material as well as an electron-transport luminescent material as the luminescent material allows for the design of a wide range of materials, which in turn provides various luminous colors. However, in terms of electroluminescent efficiency, lifetimes, and so forth, it cannot be said that expectations have been met.
When devices are used in the passive-matrix line-at-a-time scanning displays, in particular, in order to attain a prescribed average luminance, peak luminance needs to be increased to very high levels. This increases the operating voltage, causing the problem of increasing power consumption as a result of power loss or the like caused by wiring resistance. Further, other problems arise, such as an increase in the cost for drive circuits and a decrease in reliability. Furthermore, devices tend to have shorter lifetimes as compared to ones used under conditions of continuous light-emission.
In addition, even with devices having high electroluminescent efficiency and relatively low operating voltages at direct current operation, when the duty ratio increases during operation, the operating voltage required to attain a prescribed average luminance is rapidly increased and also the electroluminescent efficiency itself is reduced as the operating voltage increases.
Moreover, the above-mentioned [4-{2-(naphthalene-1-yl)vinyl}phenyl]bis(4-methoxyphenyl)amine and [4-(2,2-diphenylvinyl)phenyl]bis(4-methoxyphenyl)amine, disclosed in Japanese Unexamined Patent Publication No. 2-250292, have relatively good hole-transport properties and high fluorescent yield. However, since both compounds are low-molecular-weight compounds, they suffer from the problems of low heat-resistance and particularly a short lifetime. In addition, because the compounds require luminescent dye doping, there is a problem concerning manufacturing.
The above-mentioned 4,4′-bis(2,2-diphenyl-1-vinyl)-1,1′-biphenyl, disclosed in International Patent Publication No. WO96/22273, is somewhat superior in terms of heat-resistance as compared to the above-mentioned compounds. However, since the structure of the compound is completely symmetric, the molecules easily become associated with each other, reducing electroluminescent efficiency due to microscopic crystallization and aggregation. Because of this, devices using this kind of compound are unable to obtain satisfactory lifetime when used under conditions of continuous light-emission. In addition, since the compound requires luminescent dye doping, there is a problem concerning manufacturing.
For the above-mentioned Q1-G-Q2 type compound, such as one disclosed in the 1998 MRS Spring Meeting, Symposium G2.1, besides TPD and NPD, the trimers of and the tetramers of triphenylamine have also been reported. As for their heat resistance, it has been reported that they have sufficient levels of heat resistance. However, since these compounds also have high molecular symmetry, the molecules easily become associated with each other, reducing electroluminescent efficiency due to microscopic crystallization and aggregation. Because of this, here also, devices using this kind of compound are unable to obtain satisfactory lifetimes under continuous use. Particularly when the devices are operated at high duty cycles, difficulties arise in achieving satisfactory electroluminescent efficiency and low operating voltages. In addition, since the compounds require luminescent dye doping, there is a problem concerning manufacturing.
Devices using the above-mentioned hole-transport luminescent materials disclosed in Japanese Unexamined Patent Publications No. 10-72580 and No. 11-74079 do not require luminescent dye doping, and thus are advantageous with regard to manufacturing. However, the devices have not yet achieved satisfactory electroluminescent efficiency.